An industrial band mill has a pair of wheels, usually constituting a driving wheel and a driven wheel. These wheels are arranged vertically and support a driven band saw therebetween. In a typical machine, the wheels may be up to ten feet in diameter and fifteen inches wide; and the band saw may be up to sixty feet long, sixteen inches wide, and 0.095 inches thick. The ends of the band saw are butt welded to form a continuous band, and the band saw may be provided with teeth on both sides. The wheels are "strained up", that is, moved apart to strain the band saw on the wheels; and in an industrial band mill, the strain on the wheels could be between 14,000 to 20,000 pounds. The band saw has a tensile strength of around 270,000 pounds per square inch ("psi") and travels around the wheels at speeds of up to 10,000 feet per minute. This equipment is intended for heavy-duty involving logs or large timbers, such as in sawmills, factories producing wood products, and other industrial installations.
In the manufacture and use of band saws in band mills and the like, one of the most important criteria is the proper tensioning of the band to provide a desired flexibility along the leading cutting edge of the band saw. Accordingly, band saw fabrication techniques (as well as sash gang saws) require that the linear length of the leading edge or cutting portion of the band saw be of lesser magnitude than the trailing edge remote therefrom. To provide this characteristic, the band saw is roll-tensioned in its manufacture and during its maintenance so as to stretch the material of the band saw. This roll tensioning of band saws was initiated around 1900.
As a result, the noncutting or trailing edge has a substantially convex contour, commonly referred to as a "crown". In this manner, when the band saw is appropriately strained up between the wheels, the leading cutting edge receives the initial stretching and is therefore the most taut. Ideally, the leading edge most closely approximates a straight line when properly strained up. To attain this driving blade flexure, one of the wheels is operatively connected to an air, hydraulic or mechanical straining device which keeps the blade taut but displaced in the presence of sawing variables, such as depth of cut, feed rate, species, moisture content, density, etc.
Recommended maintenance of band mills requires removal of the band saw after about four (4) hours of continuous duty for re-sharpening and re-tensioning. Tensioning of a band saw, either during original manufacture or maintenance thereof, is a critical operation requiring highly-skilled specialized craftsmen. The craftsman performing this function in a saw mill is called a "filer". The filer's skills are in the nature of trade secrets and are often handed down from one generation to another.
As an objective, the leading or cutting edge of the band saw should define a straight line. A further objective is to assure that the band saw is flush on the wheels, when the wheels are strained up on the band mill. In checking to determine whether the band saw is flush on the strained-up wheels, the filer will often tap the band saw on the wheels to locate any "hollow" sounds. The practice of some filers is to increase the amount of tension to make the edges of the band saw tighter on the wheels. As a result, however, tremendous strain is concentrated on the edges of the band saw; these strains are excessive and result in reduced flex life and, ultimately, destruction of the band saw steel.
To alleviate these inherent structural and tensioning problems, and to obtain a desired degree of useful life and reliability of the band saw, the prior art has resorted to increasing the thickness of the band saw. However, this causes a wider kerf to be cut in the timbers, which thus results in proportionately more sawdust and wasted material.
It should be recognized, moreover, that timber conservation is a critical concern. Virgin forests in the United States and other countries are rapidly vanishing. On the average, the price of finished or sawn lumber has increased by approximately 1.7% each year since 1900, while the cost of standing live trees has increased approximately 3.5% each year. Moreover, with vanishing virgin forests, the diameter of harvested trees is substantially less than it was even thirty years ago. Accordingly, there exists a long-felt but heretofore unsatisfied need to improve the efficiency of wood processing. As a rule of thumb in the lumber industry, a reduced loss of merely 0.010 inches per saw cut equals a savings of around fifty cents ($0.50) to one dollar ($1.00) per thousand board feet of lumber produced. By reducing the size of the kerf by merely 0.031 inches, the amount of wood saved (by not being reduced to sawdust) would result in an increase in lumber yield of from one to three per cent (1-3%) with no associated increase in log costs. In many large mills, this would equate to an additional profit of about $280,000.00 annually. Thus, the relative thickness of the saw blade may have an important impact on the profitability of a sawmill.
Accordingly, a further objective is to make the band saw as thin as possible. There are two reasons for this: one, there would be an associated reduction in the amount of steel used in fabricating the band saw, thus obtaining a cost and weight advantage, as well as resulting in a minimal amount of surface area exposed adjacent to the wood for heat build-up; and two, a band saw which approximates zero thickness would result in less wood removal (in the form of sawdust) generated during the cutting operations. This would provide a substantial increase in the amount of usable board feet during each cut and would translate into a substantial savings in a large volume sawmill.
As further background, the following prior art patents are interesting relative to the present invention:
______________________________________ Inventor(s) U.S. Pat. No. Year of Issue ______________________________________ Atkins 341,989 1886 Simonds 345,565 1886 Muehlen 1,786,444 1930 Anderson 2,787,299 1957 Bertini 4,160,397 1979 ______________________________________
Briefly, the Atkins '989 patent discloses a band saw having a recess in the blade to cooperate with a crowned wheel for "tracking" purposes. While Atkins' blade is hollow formed, it is not hollow formed from edge to edge. Secondly, Atkins' purpose in hollow forming is for tracking and not for the elimination of roll tensioning. Basically, roll tensioning was not in commercial use at the time of Atkins' disclosure. Additionally, the Anderson '299 patent uses a flat wheel and discloses a band saw having a uniform thickness; it is not hollow formed, and therefore it must be roll tensioned.